System and method for coherent optical inspection

ABSTRACT

A system and method for coherent optical inspection are described. In one embodiment, an illuminating beam illuminates a sample, such as a semiconductor wafer, to generate a reflected beam. A reference beam then interferes with the reflected beam to generate an interference pattern at a detector, which records the interference pattern. The interference pattern may then be compared with a comparison image to determine differences between the interference pattern and the comparison image. According to another aspect, the phase difference between the reference beam and the reflected beam may be adjusted to enhance signal contrast. Another embodiment provides for using differential interference techniques to suppress a regular pattern in the sample.

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/415,791 filed on Oct. 2, 2002, which application isincorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] The present system and method generally relate to the inspectionof surfaces to detect defects, and in particular, to an improved systemand method that is useful in detecting defects using coherent opticalinspection techniques.

BACKGROUND

[0003] Conventionally, defect detection on semiconductor wafers can bedone with either optical or electron beam inspection. Systems andmethods for inspecting semiconductor wafers for defects using opticaland electron beam inspection techniques are well known.

[0004] Optical inspection systems frequently use either microscopic typeimaging and/or the collection of the scattered energy. For themicroscopic type of optical inspection, it may be difficult to inspectdefects that generate little intensity change from the nominalstructures. For example, dark defects on a dark background are typicallydifficult to detect due to the closeness of the change in intensity inthe reflected image due to the dark defect on the dark background.

[0005] It has been found that, in some applications, defect detectioncan be improved by using phase detection rather than intensity baseddetection, because defects that create little intensity or littleintensity change typically would have a modest phase signal.

[0006] One system for defect detection using phase detection isdisclosed in U.S. Pat. No. 6,078,392, which is incorporated herein byreference in its entirety. This patent proposes a direct-to-digitalholography approach wherein a collimated reference beam is incident upona reference beam mirror at a non-normal angle, and the reference beamand an object beam that is focused at a focal plane of a digitalrecorder to form an image. This direct-to-digital holography approach,however, requires significant computational power, which may limitthroughput. Further, this approach may be cumbersome by requiring thereference beam to be incident upon a reference beam mirror at anon-normal angle.

[0007] Another patent that refers to use of digital holograms is U.S.Pat. No. 6,282,818, the disclosure of which is incorporated herein byreference in its entirety. This patent refers to a method forsimultaneous amplitude and quantitative phase contrast imaging bynumerical reconstruction of digital holograms. This approach alsorequires significant computational power, which may limit throughput.

SUMMARY

[0008] In general, the present system and method provide for enhanceddefect signal contrast for microscopic optical inspection of a sample,such as a semiconductor wafer, by using coherent optical detectiontechniques.

[0009] One embodiment of the present invention employs coherent opticaldetection such that the output is proportional to the amplitude of thelight reflected from the sample rather than the intensity of the lightreflected from the sample. In general, an interference pattern between acomplex field reflected from the sample and common reference beam isdetected and recorded. This interference pattern is then compared with acomparison image to determine differences between the interferencepattern and the comparison image.

[0010] Specifically, one implementation of this embodiment provides forinspecting a sample by illuminating at least a portion of a sample withan illumination beam to generate a reflected beam and interfering afirst reference beam and the reflected beam to generate an interferencepattern. This interference pattern is then recorded and compared with acomparison image to detect differences between the recorded interferencepattern and the comparison image. The comparison between the recordedinterference pattern and the comparison image may comprise taking thedifference of the recorded interference pattern and the comparison imageto generate a difference field or value.

[0011] Another embodiment of the present invention utilizes interferencecontrast enhancement to boost a defect signal and improve contrast ofthe recorded interference pattern. The amplitude of the reference beammay be adjusted to boost the signal for certain areas of the samplebeing inspected. Also, the phase difference between the reference beamand the complex field may be adjusted to enhance contrast.

[0012] Specifically, one implementation of this embodiment provides forinspecting a sample by illuminating at least a portion of a sample withan illumination beam to generate a reflected beam and interfering afirst reference beam and the reflected beam to generate a firstinterference pattern. The first interference pattern is then recorded.The phase of the illumination beam is then adjusted to enhance contrastbetween a first portion of the first interference pattern and a secondportion of the first interference pattern.

[0013] Another implementation of this embodiment includes interfering asecond reference beam and the reflected beam to generate a secondinterference pattern at a second detector with the second reference beamhaving a different phase than the first reference beam. The phasedifference between the first and second reference beams may be ninetydegrees. In this implementation, adjusting the phase of the referencebeam further includes adjusting the phase of the reference beam based onat least portions of the first and second interference patterns.

[0014] Another embodiment of the present system and method utilizeinterference, such as differential interference, to suppress regularpatterns in a sample to enhance a defect signal. Specifically, accordingto one implementation of this embodiment, a sample having an array ofregularly spaced features may be inspected by illuminating the samplewith an illumination beam to generate a reflected beam and laterallyseparating the reflected beam into first and second beams. The regularlyspaced features of the sample are positioned a distance d from eachother. The first and second beams are displaced from one another by adisplacement distance equal to a multiple of the distance d, the secondbeam being about 180 degrees out of phase with the first beam. The firstbeam and the second beam interfere with each other to generate aninterference pattern, which is detected. By subtracting a pattern thatis laterally shifted from the pattern of a sample, the effect of thepattern is suppressed. The interference can be performed, for example,by using division by wavefront techniques, such as Fourier filtering, ordivision by amplitude, such as shearing through polarization orbeam-splitting. The interference may be optionally performed using aNomarski layout.

[0015] Other important technical details and advantages of the presentinvention are readily apparent to one skilled in the art from thefollowing figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the present invention andfor further features and advantages, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

[0017]FIG. 1 illustrates a portion of a sample to be inspected.

[0018]FIGS. 2A and 2B illustrate signal plots detected from the sampleof FIG. 1.

[0019]FIG. 3A schematically illustrates an example imaging system inaccordance with an embodiment of the present invention.

[0020]FIG. 3B schematically illustrates details of an example embodimentof the FIG. 3A reference module.

[0021]FIG. 4 is a plot of a simulated signal at a FIG. 3A detector forthe sample of FIG. 1 as obtained using coherent detection techniques.

[0022]FIG. 5 is a plot of a simulated signal at a FIG. 3A detector forvarious phase difference conditions between a reference beam and areflected beam.

[0023]FIGS. 6A, 6B, 6C, and 6D illustrate a simulated signal plot ofarray patterns obtained using coherent detection methods.

[0024]FIG. 7 is a plot of a difference signal across an area surroundinga feature of the FIG. 6D plot with the phase of the reference beam setat 0.0 and with the phase of the reference beam set at π/2.

[0025]FIG. 8 schematically illustrates an example imaging system inaccordance with another embodiment of the present invention.

[0026]FIGS. 9A, 9B, and 9C illustrate lateral separation and subtractionaspects of pattern subtraction through differential interferencetechniques.

[0027]FIG. 10 illustrates an example mechanism for shearing a reflectedbeam according to an embodiment of the present invention.

[0028]FIGS. 11A and 11B illustrate two orientations of the polarizingelement implementation of FIG. 10.

[0029]FIGS. 12A, 12B, 12C and 12D illustrate an example of patternsubtraction with amplitude division.

DETAILED DESCRIPTION

[0030]FIG. 1 is a cross-sectional view of one embodiment of portion of asample 100. As shown, the sample 100 includes a metal layer 102 having alength L and a SiO₂ layer 104 having a thickness t disposed on the metallayer 102. A hole 106 having a width w is formed in the SiO₂ layer 104and may comprise a via or trench, for example. The hole 106 extends theentire thickness of the SiO₂ layer 104 and exposes a portion 108 of themetal layer 102. In an example embodiment, the SiO₂ layer 104 may have athickness t of about 1400 nanometers and the hole 106 may have a width wof about 300 nanometers and the metal layer 102 may have a length L. Inthis configuration, the hole 106 may be considered a “high aspect ratio”or (HAR) structure, due to the magnitude of the ratio of the depth ofthe hole 106 to the width of the aperture 106. In this example, theaspect ratio is 1400/300, or about 4.67. HAR structures may have aspectratios in the range of about 1:1 to about 12:1, and in some cases 4:1 toabout 12:1.

[0031] As a result of certain manufacturing processes, film residue (notshown), or other non-conductive matter, were left in the aperture 106and deposited on the exposed portion 108 of the metal layer 102, therebylimiting or preventing electrical connection to the exposed portion 108of the metal layer 102 through the aperture 106. This film residuedeposited on the exposed portion 108 of the metal layer 102 may comprisea defect or anomaly in the sample. Accordingly, it is desirable in someapplications to be able to detect the presence of the film residue.

[0032] When an illuminating beam illuminates the sample 100, the areaincluding the aperture 106 generates significantly lower intensity fromthe exposed portion 108. FIGS. 2A and 2B are plots of the simulatedsignal as may be seen on a charge coupled device (CCD) camera forconventional intensity-based microscopic detection. In particular, FIG.2A is a plot of the relative intensity at the sample 100 and FIG. 2B isa plot of the intensity at the CCD camera with a 0.6 numerical apertureimaging system. FIGS. 2A and 2B generally illustrate that relativelylittle intensity is being detected from the exposed area 108 of themetal layer 102 and that the signal detected from the exposed area 108will be greatly affected by the noise in the electronics and thedetector noise.

[0033] Coherent Detection

[0034]FIG. 3A is a schematic view of an example inspection and imagingsystem 300 according to one embodiment of the present invention. Thesystem 300 is configured for inspecting a sample 310, which may comprisea semiconductor wafer or other suitable object to be inspected. Asillustrated, the system 300 generally includes detectors 302, 304, anillumination source 306, and a sample 310, such as a semiconductorwafer. The detectors 302, 304 may comprise CCD cameras or other suitableimage capture devices. The detectors 302, 304 are connected to acomputer 305 or other data processing device, for storing detectedimages and performing analysis of the same. The illumination source 306outputs an illumination beam 312, which is transmitted through lens 314.In some embodiments, the illuminating source may comprise a laser beamgenerator or other narrow band light source and the illuminating beammay comprise laser light.

[0035] The illuminating beam 312 travels to a beam splitter 313. Thebeam splitter 313 may be, for example, 50% reflective. Light that isreflected from the beam splitter 313 constitutes an object beam 316 andtravels toward the sample 310 via an objective lens 320, whichcollimates the light to illuminate the sample 310 with a collimatedbeam. A portion of the light reflected from the sample 310 comprises areflected beam 328 and passes through the beam splitter 313 and animaging lens 322 for detection by one or more detectors 302, 304. Asillustrated in FIG. 3A, a component of the reflected beam 328 travels tothe detector 302 by passing through a beam splitter 330. Anothercomponent of the reflected beam 328 travels to the detector 304 byreflecting from the beam splitter 330. The beam splitter 330 may be, forexample, 50% reflective. Alternatively, the beam splitter 330 maycomprise a polarizing beam splitter.

[0036] In an alternate embodiment (not illustrated), the detector 304 isnot present and the reflected beam 328 travels to the detector 302 fordetection.

[0037] A component of the collimated illumination beam 312 passesthrough the beam splitter 313 and travels through a lens 340 to areference module 350. Details of one example reference module 350 areillustrated in FIG. 3B and are described below. In one embodiment, thecomponent of the illumination beam 312 that passes through the beamsplitter 313 has a polarization of about 45 degrees with respect to anaxis of polarizing beam splitter 360.

[0038] The reference module 350, as described in more detail below withreference to FIG. 3B, outputs a first reference beam 352 having phase Φand a second reference beam 354 having phase Φ−π. Hence, the first andsecond reference beams 352, 354 are out of phase by 180 degrees. Thefirst and second reference beams 352, 354 can alternatively be out ofphase by 90 degrees. Further, the first and second reference beams 352,354 are polarized orthogonal to each other.

[0039] The first and second reference beams 352, 354 are substantiallycollinear and travel to the beam splitter 330. The beam splitter 330transmits light having a first polarization and reflects light having apolarization orthogonal to the first polarization. Since the first andsecond reference beams 352, 354 are polarized orthogonal to each other,the beam splitter 330 reflects one of first and second reference beams352, 354 towards the detector 302 and transmits the other of the firstand second reference beams 352, 354 to the detector 304. In oneembodiment, the first reference beam 352 reflects from the beam splitter330 towards the detector 302.

[0040] Accordingly, at least a portion of the reflected beam 328 and thefirst reference beam 352 interfere with each other at the detector 302and generate a first interference pattern, which is detected by thedetector 302. The first interference pattern results from theinterference of the first reference beam 352 and at least a portion ofthe reflected beam 328.

[0041] Likewise, at least a portion of the reflected beam 329 and thesecond reference beam 354 interfere with each other at the detector 304and generate a second interference pattern, which is detected by thedetector 304. The second interference pattern results from theinterference of the second reference beam 354 and at least a portion ofthe reflected beam 329.

[0042] In operation, the first interference pattern, as detected by thedetector 302, is compared with a comparison image to detect, ordetermine, differences between the first interference pattern and thecomparison image.

[0043] For example, the comparison image may comprise an interferencepattern obtained from a comparison sample (not shown). The comparisonsample is positioned at the location of sample 310 in FIG. 3A. Next, thecomparison sample is illuminated using the illumination beam 312 togenerate a reflected beam 328 as described above. At least a portion ofthis reflected beam 328 is then interfered with the first reference beam352 to generate a comparison interference pattern at the detector 302.The detector 302 detects and records the comparison interferencepattern. This comparison interference pattern is then stored at thecomputer 305 for later comparison with the first interference pattern.

[0044] The first interference pattern is compared with the comparisonimage to detect differences between the first interference pattern andthe comparison image. Significant differences between the firstinterference pattern and the comparison image may be associated withsignificant differences in the physical structures of the comparisonsample and the sample 310. These differences in the physical structuresof the comparison sample and the sample 310 may comprise defects.

[0045] In one embodiment, computer 305 compares the first interferencepattern with the comparison image by subtracting the first interferencepattern from the comparison image to determine or detect the differencesbetween the first interference pattern and the comparison image. Inanother embodiment, computer 305 compares the first interference patternwith the comparison image by subtracting the comparison image from thefirst interference image to determine or detect the differences betweenthe first interference pattern and the comparison image.

[0046] In general, the detector 302 detects the intensity of the firstinterference pattern. The intensity of the first interference patternmay be expressed as follows, where I_(det(A)) is the intensity of thefirst interference pattern at the detector 302, E_((A)) is the amplitudeof the reflected beam 328, E_(ref(1)) is the amplitude of the firstreference beam 352, Φ_((A)) is the phase of the reflected beam 328 andΦ_(ref(1)) is the phase of the first reference beam 352.

I _(det(A)) =|E _((A)) e ^(iΦ(A)) +E _(ref(1)) e ^(i Φref(1))|²   (1)

I _(det(A)) =|E _((A)) ² |+|E _(ref(1)) ²|+2|E _((A)) ×E _(ref(1)) *|×cos(Φ_((A))−Φ_(ref(1)))   (2)

[0047] Assuming that the amplitude squared of the beam reflected fromthe comparison image (E_((B)) ²) equals the E_((A)) ² and assumingE_(ref(1)) to be constant, subtracting the intensity I_(det(B))associated with the comparison image from the I_(det(A)) associated withthe sample 310 yields the following.

I _(det(A)) −I _(det(B))=2 E _((A)) E _(ref(1))cos(Φ_((A))+Φ_(ref(1)))−2E _((B)) E _(ref(1))cos(Φ_((B))+Φ_(ref(1)))   (3)

[0048] Accordingly, the difference I_(det(A))−I_(det(B)) between thefirst interference pattern I_(det(A)) and the comparison imageI_(det(B)) is heavily dependent oncos(Φ_((A))+Φ_(ref(1)))−cos(Φ_((B))+Φ_(ref(1))), or the difference inphase change between the first interference pattern and the comparisonimage. When the difference I_(det(A))−I_(det(B)) between the firstinterference pattern I_(det(A)) and the comparison image I_(det(B))exceeds a predetermined maximum threshold, it may be concluded that asignificant structural difference exists between the sample 310 and thecomparison sample. The difference I_(det(A))−I_(det(B)) between thefirst interference pattern I_(det(A)) and the comparison imageI_(det(B)) may be referred to as the “defect signal.”

[0049] Further, it should be noted that by increasing the amplitude ofthe first reference beam A_(ref(1)), the magnitude of the defect signalmay be boosted. Moreover, adjusting the phase of the reference beam mayfurther increase the defect signal. In one embodiment, the phase of thereference beam is adjusted to provide optimal contrast between a defectand a background pattern associated with the defect. As explained below,providing optimal contrast can mean providing maximum or minimumcontrast between a defect and a background pattern associated with thedefect. In another embodiment, the phase of the reference beam isadjusted to provide contrast between a nominal structure and acorresponding defect structure.

[0050]FIG. 3B illustrates details of an example embodiment of thereference module 350 of FIG. 3A. As illustrated, the reference module350 receives as input a component of the illumination beam 312 andoutputs the first and second reference beams 352, 354. In oneembodiment, the component of the illumination beam 312 received by thereference module 350 has a 45 degree polarization relative to apolarizing cube beam splitter 360. In one embodiment, the illuminationpath to the sample includes a polarizing element (341) to control thepolarization incident to the sample and another polarizing element (342)to rotate the polarization of the return beam so it has equal intensityfor the S and P polarization component.

[0051] The polarizing cube beam splitter 360 may be constructed of twocemented right angle prisms. As illustrated, P-polarized light istransmitted, and S-polarized light is reflected 90°. Outside surfacesmay have an anti-reflection coating to reduce back reflections.Typically, no beam displacement occurs between the original andseparated beams. The reflected and transmitted beams travel throughabout the same amount of glass, so although the optical path length ofeach arm is increased, both paths are increased by the same amount. Thecubic shape of the cube beam splitter 360 makes the cube beam splittereasy to mount in some applications, thus suffering less from deformationdue to mechanical stress. The cube beam splitter 360 is polarizationsensitive and outputs an s-polarization component S-pol and aP-polarization components P-pol. The S-Polarized component of theilluminating beam 312 comprises the first reference beam 352 and theP-Polarized component of the illuminating beam 312 comprises anintermediate beam 366.

[0052] The S-Polarized component of the illuminating beam 312, whichcomprises the first reference beam 352 is reflected by the cube beamsplitter 360 and travels from the polarizing cube beam splitter 360 at90° from the angle at which the illuminating beam 312 enters the cubebeam splitter 360. The first reference beam 352 then travels to mirror362 and reflects from the mirror 362 at 90° toward cube beam splitter370. The first reference beam 352 then enters a cube beam splitter 370and exits the cube beam splitter 370 at 90° relative to the angle atwhich the first reference beam 352 enters the cube beam splitter 370.Because the first reference beam 352 is S-polarized, the first referencebeam is reflected by the cube beam splitter 370. The first referencebeam 352 exits the cube beam splitter 370 and travels toward a mirror372 and reflects from the mirror 372 at 90° relative to the angle atwhich the first reference beam 352 is incident at the mirror 372 andexits the reference module 350 toward the beam splitter 330 (FIG. 3A).

[0053] The intermediate beam 366 exits the cube beam splitter 360 andtravels toward a mirror 376. The intermediate beam 366 reflects from themirror 376 at 90° relative to the angle at which the intermediate beam366 is incident at the mirror 376 towards a phase retarder 380. Thephase retarder 380 may comprise a conventional phase retarder thatreceives the intermediate beam 366, retards the phase of theintermediate beam 366 by π, and outputs the second reference beam 354,the second reference beam 354 lagging the intermediate beam 366 by π.Thus, where the optical path lengths between beam splitters 360 and 370are substantially the same for beams 352 and 354, the second referencebeam 354 will have a phase difference of π relative to the firstreference beam 352. Phase retarders that cause a phase difference otherthan π may alternatively be employed. In one embodiment, the phaseretarder causes a phase difference of n*2n±n, where n is an integer,although other phase differences may also be employed. The secondreference beam 354 exits the phase retarder 380 and travels towards themirror 372, passing through the cube beam splitter 370. The secondreference beam 354 then reflects from the mirror 372 at 90° relative tothe angle at which the second reference beam 354 is incident at themirror 372. The second reference beam 354 exits the reference module 350toward the beam splitter 330 (FIG. 3A). As shown in FIG. 3B, the firstand second reference beams 352, 354 may exit the reference module 350 ina collinear fashion. In FIG. 3A the first and second reference beams352, 354 are shown side by side for purposes of illustration only.

[0054] In another embodiment, the detectors 302 and 304 can beconfigured to be photon detectors that will detect the integral signal.In yet another embodiment, these two detectors can also be used duringoperation to provide the servo feedback to control the reference phase.In this embodiment, for example, detector 302 will provide theinspection signal that has minimal pattern contrast, while detector 304collects a signal that is generated with the reference beam 90 or 180degrees out of phase from the inspection signal. For example, if, due toenvironment changes, the phase of the reference beam 352 changes, thischange can be detected at the detector 302 and the direction of thechange in phase can be determined using the change detected at thedetector 304. Based on the detected change in phase of the referencebeam 352, servo positioning of mirrors, such as the mirrors 372, 376,and 362 can be performed by detecting changes in the signals detected atthe detectors 302 and 304.

[0055]FIG. 4 is a plot of a simulated signal I_(det(A)) at the detector302 for the first interference pattern described above for the sample100 shown in FIG. 1. In this plot, the amplitude of the first referencebeam 352 is set to a higher value to boost the signal from the exposedportion 108 (FIG. 3A). A comparison of the plot of FIG. 4 with the plotof FIG. 2B illustrates that the signal at the exposed portion 108 isdramatically higher using the coherent method of optical inspectiondescribed above with reference to FIGS. 3A and 3B. Thus, the signal atthe exposed portion 108 is more likely to be well above the noise floorof the associated detector, such as the detector 302.

[0056] Interference Contrast Enhancement

[0057]FIG. 5 is a plot of a simulated signal at the detector 302 forvarious phase difference conditions between the first reference beam 352and the reflected beam 328. As mentioned above, and as discussed in moredetail below, adjusting the phase of the reference beam 352 to changethe phase difference between the first reference beam 352 and thereflected beam 328 can enhance contrast of a first area, such as adefect area, of the first interference pattern relative to a second areaof the first interference pattern. As shown in FIG. 5 different phasedifferences between the first reference beam 352 and the reflected beam328 may produce significantly different signals at the detector 302.

[0058]FIGS. 6A, 6B, 6C, and 6D illustrate a simulated signal plot ofarray patterns obtained using the coherent detection methods describedabove with reference to FIGS. 3A and 3B. In each of the FIGS. 6A, 6B,6C, and 6D a portion 602 of a sample is illustrated as having an arrayof contact holes 604 and therefore comprises a comparison or a referencepattern.

[0059] In FIGS. 6A and 6C, no film residue is disposed in any of theholes 604. In FIGS. 6B and 6D, hole 606 (at array index 3,5) has filmresidue (not shown) disposed therein. The film residue in this examplecreates a phase difference of π from the other contact holes 604. Thecontact holes 604 that do not have the film residue disposed thereinexhibit a phase difference of n*π+π/2 relative to the background 608,where n comprises an integer. The contact holes 604 other than the hole606 may be referred to as “nominal structures.” As discussed below, thehole 606 in some of the FIGS. 6A, 6B, 6C, and 6D will exhibit little orno signal difference on detector 302 from the nominal structures (i.e.,the other holes 604).

[0060] In the plots of FIGS. 6A and 6B, the phase of the first referencebeam 352 is set to create high contrast between the background and thepattern of holes. Despite there being film residue in the hole 606,however, no significant signal difference is present between the plotsof FIGS. 6A and 6B. Hence, in FIGS. 6A and 6B, there is low signalcontrast in the defect area (i.e., at hole 606).

[0061] In FIGS. 6C and 6D, however, the phase of the reference beam isadjusted by a phase difference of π from the background 608, whichenhances the signal contrast at hole 606, but may decrease contrastbetween the background and the pattern of holes. As shown, in FIGS. 6Cand 6D, the difference signal between the holes 604 and the background608 is less than that of FIGS. 6A and 6B. Importantly, however, thesignal difference in the area of the hole 606 in the plot of FIG. 6D ishigh and is easily detected by comparison with the plot of FIG. 6C.

[0062] The example of FIGS. 6A, 6B, 6C, and 6D illustrates the advantageof using a second reference beam that differs in phase from a firstreference beam by π. As shown above in FIGS. 3A and 3B, the secondreference beam 354 differs in phase from the first reference beam 352 byπ. The second reference beam 354 interferes with the reflected beam 328and generates a second interference pattern at the detector 304.Depending on the phase values of the various structures, or areas, ofthe sample being inspected, adjusting the phase of the reference beammay significantly improve contrast between a defect and the backgroundpattern, between a defect structure and a nominal structure, or both.

[0063] In one embodiment, the phase of the reference beam is adjustedbased on the first interference pattern detected at the detector 302 andthe second interference pattern detected at the detector 304.

[0064] For array high aspect ratio inspection, in one embodiment, thephase for the reference beam may be adjusted so it results in minimalcontrast for the array pattern. This setting would enhance the contrastbetween any anomaly and the background. The necessary phase setting forthe reference beam can be determined based on the interference patterndetected at the detector 302 and the second interference patterndetected at the detector 304. For example,

I ₁ =I _(B) +I _(R)+2{square root}{square root over (I_(B)I_(R))} cos(φ_(B)−φ_(R))≈2I _(R)+2I _(R) cos(φ_(B)−φ_(R))

I ₂ =I _(H) +I _(R)+2{square root}{square root over (I_(H)I_(R))} cos(φ_(H)−φ_(R))≈I _(R)

[0065] where I_(B) is the intensity from the background,

[0066] I_(H) is the intensity from the high aspect area and is typicallysignficantly lower than I_(B)

[0067] I_(R) is the intensity from thereference beam, and is typicallyset to be equal to I_(B)

[0068] I₁ is the resulting interference signal from the background,while I₂ is the one from the high aspect area.

[0069] In order to have minimal contrast for the array pattern, I₁ shallbe similar to I₂. This condition can be met when the phase of thereference beam φ_(R) is set so cos(φ_(B)−φ_(R))˜−0.5. To set this φ_(R)value, first we can acquire I₁ at any phase setting for the referencebeam, φ_(R1), and also 180 degrees out of phase. These two data sets canbe taken sequentially or taken simultaneously as outlined in FIG. 3Awith detectors 302 and 304 during pre-scan. The optimal φ_(R) can bethen determined from the data as follows.

I ₁(R1)=2I _(R)+2I _(R) cos(φ_(B)−φ_(R1))

I ₁(R2)=2I _(R)+2I _(R) cos(φ_(B)−φ_(R1)−π)

I _(S) ≡I ₁(R1)+I ₁(R2)=2I _(R)(2+2 cos(φ_(B)−φ_(R1)−π/2))

I _(D) ≡I ₁(R1)−I ₁(R2)=2I _(R)(2 sin(φ_(B)−φ_(R1)−π/2)sin(−π/2))

I _(D) /I _(S)≅−sin(φ_(B)−φ_(R1)−π/2)≅cos(φ_(B)−φ_(R1))

[0070] The optimal reference phase can be set by adding an additionalphase of cos⁻¹(I_(D)/I_(S))+[(2n+1)π±1/3π] to the reference beam where nis an integer.

[0071] In another embodiment, the phase of the reference beam can be setto maximize the contrast between the defected area and the nominalpattern. For example, for two similar objects A, B where A representsthe nominal pattern and B represents defect, the interference signals atthe detector are

I _(A) =I ₀ +I ₁ cos(φ_(A)−φ_(R))

I _(B) =I ₀ +I ₁ cos(φ_(B)−φ_(R))${{\Delta \quad I} \equiv {I_{A} - I_{B}}} = {I_{1}\left( {2\sin \frac{1}{2}\left( {\varphi_{A} + \varphi_{B} - {2\quad \varphi_{R}}} \right)\sin \frac{1}{2}\left( {\varphi_{B} - \varphi_{A}} \right)} \right)}$

[0072] For ΔI to be maximum,$\frac{\left( {\Delta \quad I} \right)}{\varphi_{R}} \approx 0$

[0073] Since$\frac{\left( {\Delta \quad I} \right)}{\varphi_{R}} \cong {2\quad I_{1}\sin \frac{1}{2}\left( {\varphi_{B} - \varphi_{A}} \right)\cos \frac{1}{2}\left( {\varphi_{A} + \varphi_{B} - {2\quad \varphi_{R}}} \right)\left( {- 1} \right)} \approx 0$

$\cos \frac{1}{2}\left( {\varphi_{A} + \varphi_{B} - {2\quad \varphi_{R}}} \right)$

[0074] shall be equal to${\left( {n + \frac{1}{2}} \right)\quad \pi},$

[0075] where n is an integer This means that when the reference phase isset to be${\frac{\varphi_{A} + \varphi_{B}}{2} - {\left( {n + \frac{1}{2}} \right)\quad \pi}},$

[0076] the contrast between the interference signals from patterns A & Bis maximal.

[0077]FIG. 7 is a plot of a difference signal across the areasurrounding the hole 606 of FIG. 6D with the phase of the firstreference beam 352 (FIG. 3) set at 0.0 and the phase of the secondreference beam 354 set at π/2. In the plot of FIG. 7, the phase of thenominal contact hole 604 is at 0.4 π, rather than at 0.5 π as used abovewith reference to FIGS. 6A-6D. As shown in FIG. 7, the second referencebeam 354 causes a significantly higher difference in the relative signalthan the first reference beam 352, in this example.

[0078] Pattern Subtraction through Interference

[0079] As mentioned above, one challenge associated with inspection of asample, such as a semiconductor wafer, is detecting film residue, orother matter, at a bottom portion of a high aspect ratio structure, suchas a hole or trench. In some applications, the sample includes arepeating array of high aspect ratio structures in a pattern. Oneexample of such a sample is the portion 602 of the sample shown in FIG.6A, which includes a repeating array of contact holes 604.

[0080] For samples that include such a repeating pattern of structures,it may be desirable in some applications to remove, or suppress, theregular, nominal pattern from the analysis to enhance the defect area.According to one embodiment, differential interference is used tomeasure the difference between a defect pattern and a nominal pattern.The interference may be accomplished, for example, using division bywavefront interference, such as by using Fourier filtering. Theinterference may also be accomplished by division by amplitudeinterference, such as by shearing through polarization orbeam-splitting.

[0081]FIG. 8 is a schematic view of an example inspection and imagingsystem 800 according to one embodiment of the present invention forinspecting the sample 310 having a repeating array of structures withadjacent structures being separated by a distance d as measured at thedetector. In one embodiment, the sample 310 may comprise a semiconductorwafer having an array of contact holes. As shown in FIG. 8, imagingsystem 800 is identical to the imaging system 300 shown in FIG. 3A anddescribed above, except as follows.

[0082] The imaging system 800 includes polarizing elements 802positioned between the beam splitter 313 and the imaging lens 322. Thepolarizing elements 802 generally receive the reflected beam 328,separate the reflected beam 328 into first and second laterallyseparated beams 806, 808 and phase shift the second laterally separatedbeam 808 by π, or 180 degrees. The polarizing elements 802 laterallyshift the beams 806, 808 such that they are laterally separated by amultiple of the distance d at the detector 302, where the distance d isthe distance between adjacent structures as measured at the detector 302of a repeating array of structures of the sample 310. The beams 806, 808then interfere with each other at the detector 302 where the beams 806,808 are laterally offset by the distance d. Optionally, the firstreference beam 352 also interferes with the first and second laterallyshifted beams at the detector 302.

[0083]FIG. 8 illustrates two detectors 302 and 304. In some embodiments,however, only a single detector is employed.

[0084] Details regarding some embodiments of the polarizing elements 802that laterally separate a beam and introduce a phase shift into one ofthe laterally separated beams are well-known and are described in“Optical Interferometry” by M. Francon (ISBN 0122663500), the disclosureof which is hereby incorporated by reference. Additional detailsregarding one embodiment of the polarizing elements 802 are describedbelow with reference to FIGS. 9, 10, 11A, and 11B.

[0085] Thus, the beam 806 is associated with a set of the repeatingarray of structures and the beam 808 is associated with the same set ofthe repeating array of structures. The beams are offset by a distanceequal to a multiple of the distance d. Interfering the beams 806, 808 inthis manner causes repeating structures in the pattern or imageassociated with the beam 808 to be subtracted from repeating structuresin the pattern or image associated with the beam 806.

[0086]FIGS. 9A, 9B, and 9C illustrate lateral separation and subtractionaspects described above in accordance with one embodiment. As shown inFIG. 9A, a pattern 902 detected at detector 302 (FIG. 8) from a sample,such as the sample 310, includes an array of periodically repeatingfeatures 904, which are separated by a distance d, which is also thepitch of pattern 902. The sample 310 may comprise a portion of asemiconductor wafer and the each of the features 904 may comprise acontact hole formed in the semiconductor wafer 902. The features 904 mayeach alternatively comprise another HAR structure or a non-HARstructure. Feature 906 is a particular one of the features 904, locatedat position 3,3, and includes a defect.

[0087] Using the imaging system 800 described above, the illuminatingbeam 316 illuminates the sample 310 (FIG. 8) generating reflected beam328. The reflected beam 328 enters the polarizing elements 802. Thepolarizing elements 802 then laterally separate the reflected beam 328into beams 806, 808, such that the beam 808 is 180 degrees out of phaseand laterally separated from the beam 806. The beams 806, 808 then passthrough imaging lens 322 and are transmitted to at least one of thedetectors 302, 304.

[0088] The beams 806, 808 interfere at at least one of the detectors302, 304 such that they generate a first pattern 902 and a secondpattern 914 (FIG. 9B) with periodically repeating features offset bydistance d (shown in dashed lines). The first and second patterns 902and 914 interfere with each other. The first pattern 902 shows thedefect 906 and the second pattern 914 shows the defect 906′. The defect906′ is 180 degrees out of phase with the defect 906 and is laterallyoffset by the distance d. The first and second patterns 902 and 914 arelaterally offset by the distance d at the detectors 302, 304 and are 180degrees out of phase relative to each other, such that the first andsecond patterns 902, 914 destructively interfere to generate adifference pattern 916 (FIG. 9C). As shown, interfering the patterns 902and 914 results in the subtraction of repeating background pattern andhighlights the presence of the defect 906 in the difference pattern 916.The difference pattern 916 illustrates the defect 906 without thebackground comprising the pattern of features 904. By interfering thebeams 806 and 808, the background pattern is substantially removed, thuspermitting enhanced detection of the defect 906.

[0089]FIG. 10 illustrates one embodiment of a mechanism 1000 forshearing the reflected beam 328 as performed by the polarizing elements802 (FIG. 8). As shown, the mechanism 1000 includes a pair of Wollastonprisms 1002 and 1004, where θ is the separation angle of a prism and λis the wavelength of the reflected beam 328.

[0090] In general, a Wollaston prism typically includes two wedges ofquartz, calcitite, or other suitable birefringent or doubly-refractingmaterial, cut in such a way that their optical axes are orientedperpendicular when they are cemented together to form a prism. Lightentering the Wollaston prism is split into two beams such that a phasedifference between the two beams is created. Because the two beams areeach derived from the same source prior to being sheared by theWollaston prism, they are coherent and are capable of interference.

[0091] Referring back to FIG. 10, the prisms 1002 and 1004 are arrangedsuch that the reflected beam 328 enters the prism 1002, is split into apair of beams that have polarization vectors mutually perpendicular toeach other. This pair of beams then pass through prism 1004. As thereflected beam 328 passes through the prism 1002, the prism 1002 shears,or separates, the beam into a pair of beams, with one of the beams beingphase-shifted relative to the other beam. These beams then pass throughprism 1004 where they are further displaced relative to each other.

[0092]FIGS. 1A and 1B illustrate two orientations of the polarizingelement implementation of FIG. 10. FIG. 11A shows a zero-degreeconfiguration of a polarizing element 1100 that includes Wollastonprisms 1102 and 1104, which will shear, or separate, the reflected beam328 to the maximum displacement of the sheared beams. The displacementdepends on the angle separation of the Wollaston prism used. FIG. 11Bshows a 90-degree configuration of a polarizing element 1200 thatincludes Wollaston prisms 1202 and 1204, which shear the reflected beam328 to a minimum separation between the sheared beams. The amount ofbeam separation imposed by the Wollaston prisms may be adjusted betweenthe zero-degree configuration of FIG. 11A and the 90-degreeconfiguration of FIG. 11B to create beam shearing sufficient to providean amount of beam separation appropriate for the pattern subtractionthrough interference described above.

[0093]FIGS. 12A and 12B illustrate an example of pattern subtractionwith amplitude division. In this example, a three-bar pattern that has areflected amplitude ratio of 0.5 between the bottom of the structure andthe top of the structure was simulated. FIG. 12A is the plot of theintensity profile of the simulated object. FIG. 12B shows the phaseprofile for two different patterns: one has a smaller phase at the rightend of the bar and the other has a smaller phase at the left endinstead. The inspection task is to compare these two structures andpickup the two phase differences at the two ends.

[0094] In this differential interference, the phase of the secondpattern 1222 is shifted 180 degrees relative to the first pattern 1220,and this generates the subtraction effect between the two patterns. FIG.12C illustrates the signal difference at the detector, such as detector302 (FIG. 8) through differential interference detection of the objectof FIG. 12A. FIG. 12D illustrates the signal difference for the objectof FIG. 12A with a 0.1 amplitude ratio. Thus, by removing or suppressingthe background pattern, significant signal contrast can be obtained.Further, FIGS. 12C and 12D demonstrate that with coherent detection, theimpact from electronics noise may be reduced or minimized.

[0095] While various embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that numerousalterations may be made without departing from the inventive conceptspresented herein. Thus, the invention is not to be limited except inaccordance with the following claims and their equivalents.

What is claimed is:
 1. A method for inspecting a sample, the methodcomprising: illuminating at least a portion of a sample with anillumination beam to generate a reflected beam; interfering a firstreference beam and the reflected beam to generate an interferencepattern; recording the interference pattern; comparing the recordedinterference pattern with a comparison image to detect differencesbetween the recorded interference pattern and the comparison image. 2.The method for inspecting a sample according to claim 1, wherein thecomparing further comprises subtracting the recorded interferencepattern from the comparison image.
 3. The method for inspecting a sampleaccording to claim 1, wherein the comparing further comprisessubtracting the comparison image from the recorded interference pattern.4. The method for inspecting a sample according to claim 1, wherein thesample comprises a portion of a wafer having a repeatable array offeatures.
 5. The method for inspecting a sample according to claim 1,further comprising determining whether differences between the recordedinterference pattern and the comparison image exceed a predeterminedthreshold.
 6. The method for inspecting the sample according to claim 1,wherein the reference beam and the illumination beam have a commonphase.
 7. The method for inspecting the sample according to claim 1,wherein a component of the illumination beam the reference beam passesthrough a beam splitter, the component of the illumination beamcomprising the reference beam.
 8. The method for inspecting the sampleaccording to claim 1, wherein the reference beam and the illuminationbeam have a common source.
 9. The method for inspecting the sampleaccording to claim 1, wherein the reference beam reflects from a beamsplitter before interfering with the first image.
 10. The method forinspecting the sample according to claim 1, wherein the sample includesHigh Aspect Ratio (HAR) structures.
 11. The method for inspecting thesample according to claim 10, wherein the HAR structures have aspectratios in the range of about 1:1 to about 12:1.
 12. A method forinspecting a sample, the method comprising: illuminating at least aportion of a sample with an illumination beam to generate a reflectedbeam; interfering a first reference beam and the reflected beam togenerate a first interference pattern; recording the first interferencepattern; adjusting a phase of the illumination beam to adjust contrastbetween a first portion of the first interference pattern and a secondportion of the first interference pattern.
 13. The method for inspectinga sample according to claim 12, wherein the adjusting of the contrastprovides optimal contrast between the first portion of the firstinterference pattern and the second portion of the first interferencepattern.
 14. The method for inspecting a sample according to claim 12,further comprising: determining a first average intensity value for thefirst portion of the first interference pattern and a second averageintensity value for the second portion of the first interferencepattern; determining a first interference pattern difference value basedon the difference between the first and second average intensity valuesfor the first interference pattern; interfering a second reference beamand the reflected beam to generate a second interference pattern, thesecond reference beam having a different phase than the first referencebeam; determining a first average intensity value for a first portion ofthe second interference pattern and a second average intensity value forthe second portion of the second interference pattern; determining asecond interference pattern difference value based on the differencebetween the first and second average intensity values for the secondinterference pattern; wherein the adjusting the phase of the referencebeam further comprises adjusting the phase of the reference beam basedon the first and second interference pattern difference values.
 15. Themethod for inspecting a sample according to claim 14, wherein the secondreference beam and the first reference beam are 180 degrees out of phasewith each other.
 16. The method for inspecting a sample according toclaim 14, wherein the adjusting the phase of the illumination beamfurther comprises adjusting the phase of the illumination beam based ona ratio of the first and second interference pattern difference values.17. The method for inspecting a sample according to claim 12, furthercomprising interfering a second reference beam and the reflected beam togenerate a second interference pattern, the second reference beam havinga different phase than the first reference beam; wherein the adjustingthe phase of the reference beam further comprises adjusting the phase ofthe reference beam based on at least portions of the first and secondinterference patterns.
 18. The method for inspecting a sample accordingto claim 17, further comprising: detecting the first interferencepattern at a first detector; detecting the second interference patternat a second detector.
 19. The method for inspecting a sample accordingto claim 17, wherein the first and second reference beams are polarizedorthogonal to each other.
 20. The method for inspecting a sampleaccording to claim 17, wherein the first reference beam has a same phaseas the illumination beam and the second reference beam is ninety degreesout of phase with the illumination beam.
 21. The method for inspecting asample according to claim 17, further comprising: passing a component ofthe illumination beam passes through a first polarizing beam splitter,the component of the illumination beam having a polarization of about 45degrees; passing the component of the illumination beam through a secondpolarizing beam splitter to generate the first reference beam having afirst polarization and an intermediate beam having a secondpolarization, the first and second polarizations being orthogonal toeach other; transmitting the intermediate beam through a phase retarderto generate the second reference beam, the second reference beam havinga phase that substantially differs from the phase of the first referencebeam.
 22. The method for inspecting a sample according to claim 17,wherein the wherein the first and second reference beams are polarizedorthogonal relative to each other, one of the first and second referencebeams reflecting from a beam splitter before interfering with thereflected beam and the other of the first and second reference beamspropagating through the beam splitter before interfering the withreflected beam.
 23. The method for inspecting a sample according toclaim 17, further comprising: receiving a component of the illuminationbeam at a first polarizing beam splitter to generate the first referencebeam having a first polarization and an intermediate beam having asecond polarization, the first and second polarizations being orthogonalto each other; passing the intermediate beam through a phase retarder togenerate the second reference beam, the second reference beam having aphase that substantially differs from the phase of the first referencebeam.
 24. A method for inspecting a sample, the method comprising:illuminating a sample with an illumination beam to generate a reflectedbeam, the sample comprising an array of spaced features, with a distanced between adjacent features; laterally separating the reflected intofirst and second beams, the first and second beams being displaced fromone another by a displacement distance equal to a multiple of thedistance d, the second beam being about 180 degrees out of phase withthe first beam; interfering the first beam and the second beam togenerate an interference pattern; detecting the interference pattern.25. The method for inspecting a sample according to claim 24, whereinthe interfering further comprises interfering a reference beam with thefirst and second beams to generate the interference pattern.
 26. Themethod for inspecting a sample according to claim 24, wherein theinterfering further comprises division amplitude interference.
 27. Themethod for inspecting a sample according to claim 24 wherein theinterfering further comprises division amplitude interference throughpolarization.
 28. The method for inspecting a sample according to claim24, wherein the interfering further comprises division wavefrontinterference through Fourier filtering.
 29. The method for inspecting asample according to claim 24, wherein the interfering further comprisesdivision wavefront interference.
 30. The method for inspecting a sampleaccording to claim 24 wherein the interfering further comprises using aNomarski layout.
 31. The method for inspecting a sample according toclaim 24, wherein the interfering the first beam and the secondsubtracts the second beam from the first beam to form the interferenceimage.
 32. The method for inspecting a sample according to claim 19,wherein the displacement distance equals the distance d.
 33. The methodfor inspecting a sample according to claim 19, wherein the displacementdistance equals twice the distance d.
 34. A method for inspecting asample, the method comprising: illuminating at least a portion of asample with an illumination beam to generate a reflected beam;interfering a first reference beam and the reflected beam to generate aninterference pattern; recording the interference pattern; comparing therecorded interference pattern with a comparison image to detectdifferences between the recorded interference pattern and the comparisonimage; adjusting a phase difference between the reflected beam and thefirst reference beam to adjust contrast between a first portion of theinterference pattern and a second portion of the interference pattern.35. The method for inspecting a sample according to claim 34, whereinthe adjusting of the contrast provides optimal contrast between thefirst portion of the first interference pattern and the second portionof the first interference pattern.
 36. The method for inspecting thesample according to claim 34, wherein the reference beam and theillumination beam have a common illumination source.
 37. The method forinspecting the sample according to claim 34, wherein a component of theillumination beam the reference beam passes through a beam splitter, thecomponent of the illumination beam comprising the reference beam. 38.The method for inspecting the sample according to claim 34, wherein thereference beam and the illumination beam have a common source.
 39. Themethod for inspecting the sample according to claim 34, wherein thereference beam reflects from a beam splitter before interfering with thefirst image.
 40. The method for inspecting a sample according to claim34, further comprising: determining a first average intensity value forthe first portion of the first interference pattern and a second averageintensity value for the second portion of the first interferencepattern; determining a first interference pattern difference value basedon the difference between the first and second average intensity valuesfor the first interference pattern; interfering a second reference beamand the reflected beam to generate a second interference pattern, thesecond reference beam having a different phase than the first referencebeam; determining a first average intensity value for a first portion ofthe second interference pattern and a second average intensity value forthe second portion of the second interference pattern; determining asecond interference pattern difference value based on the differencebetween the first and second average intensity values for the secondinterference pattern; wherein the adjusting the phase of theillumination beam further comprises adjusting the phase of theillumination beam based on the first and second interference patterndifference values.
 41. The method for inspecting a sample according toclaim 40, further comprising interfering a second reference beam and thereflected beam to generate a second interference pattern, the secondreference beam having a different phase than the first reference beam;wherein the adjusting the phase of the reference beam further comprisesadjusting the phase of the reference beam based on at least portions ofthe first and second interference patterns.
 42. The method forinspecting a sample according to claim 41, further comprising: detectingthe first interference pattern at a first detector; detecting the secondinterference pattern at a second detector.
 43. The method for inspectinga sample according to claim 41, wherein the first reference beam has asame phase as the illumination beam and the second reference beam is 180degrees out of phase with the illumination beam.
 44. The method forinspecting a sample according to claim 41, further comprising: passing acomponent of the illumination beam passes through a first polarizingbeam splitter, the component of the illumination beam having apolarization of about 45 degrees; passing the component of theillumination beam through a second polarizing beam splitter to generatethe first reference beam having a first polarization and an intermediatebeam having a second polarization, the first and second polarizationsbeing orthogonal to each other; transmitting the intermediate beamthrough a phase retarder to generate the second reference beam, thesecond reference beam having a phase that substantially differs from thephase of the first reference beam.
 45. A inspection apparatus forinspecting a sample, the apparatus comprising: an illumination sourcefor providing an illumination beam at the sample to generate a reflectedbeam; a reference module for providing first and second reference beams,the first and second reference beams being out of phase with each other;a first detector aligned to detect a first interference patterngenerated by at least a component of the reflected beam and the firstreference beam; a second detector aligned to detect a secondinterference pattern generated by at least a component of the reflectedbeam and the second reference beam.
 46. The inspection apparatus ofclaim 45, wherein the first and second reference beams differ in phaseby 180 or 90 degrees.
 47. The inspection apparatus of claim 45, furthercomprising a beam splitter for reflecting one component of theillumination beam at the sample and for permitting at least anothercomponent of the illumination beam to pass through the beam splitter tothe reference module.
 48. The inspection apparatus of claim 45, whereinthe reference module further comprises a phase retarder disposed alongan optical path of one of the two reference beams.
 49. The inspectionapparatus of claim 45, wherein the reference module further comprises: afirst polarizing beam splitter for receiving at least a component of theillumination beam and separating the component of the illumination beaminto the first reference beam and an intermediate beam; a phase retarderfor changing the phase of the intermediate beam to generate the secondreference beam.
 50. The inspection apparatus of claim 45, wherein thefirst and second reference beams are orthogonally polarized relative toeach other.
 51. The inspection apparatus of claim 45, further comprisingone or more polarizing elements in optical paths of the illumination andreflected beams, to direct polarized light to the sample and to rotatepolarization of the reflected beam so that it has equal intensityorthogonal optical components.
 52. The inspection apparatus of claim 45,further comprising polarizing elements disposed along an optical pathassociated with the reflected beam for separating the reflected beaminto first and second beams, the first and second beams being laterallyseparated and 180 degrees out of phase relative to each other.
 53. Theinspection apparatus of claim 52, wherein the polarizing elementsfurther comprise a pair of aligned Wollaston prisms.
 54. An inspectionapparatus for inspecting a sample having an array of features, each ofthe features being separated by certain distance from an adjacentfeature, the inspection apparatus comprising: an illumination source forproviding an illumination beam at the sample to generate a reflectedbeam; a polarizing element positioned to receive the reflected beam andconfigured to separate the reflected beam into first and second beams,the first and second beams being laterally separated and 180 degrees outof phase relative to each other; a first detector aligned to detect afirst interference pattern generated by interference of the first andsecond beams.
 55. The inspection apparatus of claim 54, furthercomprising a reference module for generating a first reference beam, thefirst reference beam interfering with the first and second beams at thefirst detector.
 56. The inspection apparatus of claim 54, wherein thepolarizing element comprises at least one Wollaston prism.
 57. Theinspection apparatus of claim 54, further comprising: a reference modulefor generating first and second reference beams, the first referencebeam interfering with the first and second beams at the first detector;a second detector aligned to detect a second interference patterngenerated by interference of the first beam, the second beam, and thesecond reference beam.